1. Field of the Invention
The present invention relates to liquid crystal panels for liquid crystal display (LCD) devices. More particularly, the present invention relates to a liquid crystal panel that uses a ferroelectric liquid crystal.
2. Discussion of the Related Art
A conventional liquid crystal display (LCD) includes a display panel. A display panel typically has upper and lower substrates that are attached with each other, and an interposed liquid crystal, usually a nematic, a smetic, or a cholesteric liquid crystal. A liquid crystal display device utilizes the electro-optic effects of the liquid crystal. A display panel is operationally divided into a plurality of liquid crystal cells. On the exterior surfaces of the upper and lower substrates, polarizers or retardation films are selectively attached.
A major design consideration of a liquid crystal cell is the characteristics of the particular liquid crystal that is used. A good liquid crystal should have a fast response time, a good gray scale, and a wide viewing angle, all while operating at a low driving voltage. However, it is very difficult to find a liquid crystal that has all of those characteristics. Thus, various designs have been adopted for liquid crystal display devices.
Among the various types of liquid crystals, a low twisted nematic (LTN) liquid crystal has advantages of a short response time and a good gray scale. However, it typically has low contrast ratios and relatively poor color-dispersion properties. Other twisted nematic (TN) liquid crystals have higher twist angles (such as 90 degrees) or employing an in-plane switching (IPS) mode. While those liquid crystals can provide a wide viewing angle, afterimages are produced when displaying moving images, and their brightness is relatively low. The anti-ferroelectric liquid crystal (AFLC), or the optically compensated birefringence (OCB), have advantages of a wide viewing angle and a fast response time, although there are problems with contrast ratios and cell gap control.
FIG. 1 is a cross-sectional view illustrating a conventional TN-LCD panel 20. As shown in FIG. 1, the TN-LCD panel has lower and upper substrates 2 and 4 and an interposed liquid crystal layer 10. The lower substrate 2 includes a substrate 1 having a TFT “S” that is used as a switching element to change the orientation of the liquid crystal molecules. The TFT “S” includes a pixel electrode 14 that applies a voltage to the liquid crystal layer 10 in accordance with signals that are applied to the TFT “S”. The upper substrate 4 has a color filter 8 for implementing color, and a common electrode 12 on the color filter 8. The common electrode 12 serves as an electrode for applying a voltage to the liquid crystal layer 10. The pixel electrode 14 is arranged over a pixel portion “P,” i.e., a display area. Further, to prevent leakage of the liquid crystal layer 10 between the substrates 2 and 4, those substrates are sealed by a sealant 6.
FIGS. 2A to 2C illustrate various alignments of possible liquid crystal molecules in the liquid crystal layer. As shown in FIG. 2A, in the nematic liquid crystal, each rod-like molecule fluctuates quite rapidly, but the molecules have a definite orientational order expressed by a unit vector “” called a director. As shown in FIG. 2B, in the smetic liquid crystal the molecules have a layered structure in which the molecular orientation is perpendicular or nearly perpendicular to the layers. As shown in FIG. 2C, in the cholesteric liquid crystal, the director  changes its orientation gradually along a helical axis. The helical axis coincides with the optical axis of this material. Among the three different types of liquid crystals, the nematic liquid crystal is most widely used in liquid crystal display devices.
Liquid crystals for liquid crystal display devices should:                a) have a liquid crystal phase that extends from low to high temperatures, and thus are operable over a range of temperatures;        b) be chemically and optically stable over time;        c) have a low viscosity and a fast response time;        d) have highly ordered molecular alignments and thus provide a good contrast; and        e) have a large dielectric anisotropy and a low operating voltage.        
The electro-optic effect enables electrical modulation of light by changing the alignment of the liquid crystal molecules using an applied electric field.
Among the various types of nematic liquid crystals, a twisted nematic (TN) liquid crystal and a super twisted nematic (STN) liquid crystal are often used. For a TN liquid crystal panel, a nematic liquid crystal is interposed between transparent lower and upper electrodes (reference the common electrode 12 and the pixel electrode 14 of FIG. 1). Those electrodes induce a definite molecular arrangement such that a gradual rotation of the molecules occurs between the lower transparent electrode and the upper transparent electrode until a twist angle of 90 degrees is achieved. In an STN liquid crystal panel the angle of twist rotation is increased to 180 to 360 degrees.
The basic configuration and operation of a twisted nematic liquid crystal display device will now be explained. As shown in FIG. 3A, opposed and spaced apart first and second polarizers 10 and 16, respectively, have perpendicular first and second transmittance axis directions 40 and 42. Between the two polarizers 10 and 16 are first and second transparent substrates 12 and 14, which are also opposed to and spaced apart from each other. Spacers are used to maintain the cell gap between the substrates. For example, plastic balls or silica balls having a diameter of 4 to 5 micrometers can be sprayed on the first substrate.
Still referring to FIG. 3A, the first and the second transparent substrates 12 and 14 include first and second orientation films 20 and 22, respectively, on their opposing surfaces. Between the first and second orientation films 20 and 22 is a positive TN liquid crystal 18.
The positive TN liquid crystal 18 has a characteristic that it arranges according to an applied electric field. The first and second polarizer 10 and 16, respectively, transmit light that is parallel with their transmittance-axis directions 40 and 42, but reflect or absorb light that is perpendicular to their transmittance-axis directions 40 and 42.
The first and second orientation films 20 and 22 were previously rubbed in a proper direction with a fabric. This rubbing causes the positive TN liquid crystal molecules between the first and second transparent substrates 12 and 14 to become tilted several degrees. First and second rubbing directions 50 and 52 of the first and second orientation films 20 and 22 are, respectively, parallel with the transmittance-axis directions of the first and second polarizers 10 and 16. With no electric field applied across the positive TN liquid crystal 18, the orientation of the liquid crystal molecules twists between one substrate to the other at a definite angle, that angle being the twisted angle of the positive TN liquid crystal 18.
During operation, a back light device 24 irradiates white light onto the first polarizer 10. The first polarizer 10 transmits only the portion of the light that is parallel with the first transmittance-axis direction 40. The result is a first linearly polarized light 26 that passes through the polarizer 10. The first linearly polarized light 26 then passes through the positive TN liquid crystal 18 via the first transparent substrate 12.
As the first polarized light 26 passes through the positive TN liquid crystal 18, the first polarized light 26 changes its phase according to the twisted alignment of the positive TN liquid crystal molecules. Accordingly, the first linearly polarized light 26 becomes an elliptically (possibly circularly) polarized light 28.
The elliptically polarized light 28 passes through the second transparent substrate 14, and meets the second polarizer 16. When the elliptically polarized light 28 passes through the second polarizer 16, the second polarizer 16 transmits only the portion of the elliptically polarized light 28 that is parallel to the second transmittance-axis direction 42. A polarized light 30 is then emitted. In the above-mentioned operation, a white state is displayed.
Turning now to FIG. 3B, when a voltage supplier 35 induces an electric field through the positive TN liquid crystal 18, the positive TN liquid crystal molecules rotate and arrange such that the longitudinal axes of the molecules are perpendicular to the surfaces of the first and second substrates 12 and 14. Accordingly, the first linearly polarized light 26 passes through the first transparent substrate 12, the positive TN liquid crystal 18, and the second transparent substrate 14 without phase change. The first linearly polarized light 26 then meets the second polarizer 16. As the second polarizer 16 has the second transmittance-axis direction 52 which is perpendicular to the first linearly polarized light 26, the second polarizer 16 absorbs or shields most of the first linearly polarized light 26. Thus, little or none of the first linearly polarized light 26 passes through the second polarizer 16. Accordingly, a dark state is displayed.
Recently, a liquid crystal projector that uses the above-mentioned TN liquid crystal panel has been developed, although research continues. The liquid crystal projector displays images for many users in a theater or in a meeting room. In that liquid crystal projector, transmissive liquid crystal panels having TFTs are used as light valves.
Referring to FIG. 4, the liquid crystal projector includes red, green, and blue dichroic mirrors 200a, 200b, and 200c; red, green, and blue liquid crystal panels 220a, 220b, and 220c; and a lens 240 that concentrates and focuses light from the liquid crystal panels onto an image screen 250 that displays images.
In operation, a light source (not shown in FIG. 4, but see FIG. 12) irradiates white light onto the red dichroic mirror 200a. That mirror reflects the red portion of the white light to the red liquid crystal panel 220a. The green and blue portions of the white light pass through the red dichroic mirror 200a to the green dichroic mirror 200b. The green dichroic mirror 200b reflects the green portion of the white light onto the green liquid crystal panel 220b. The blue portion of the white light is directed onto the blue liquid crystal panel 220c. The red light from the red liquid crystal panel 220a is reflected by a first total reflection prism 230a into the lens 240. The green light from the green liquid crystal panel 220b is reflected by a second total reflection prism 230b into the lens 240. The blue light from the blue liquid crystal panel 220c is reflected first by a blue dichroic mirror 200c, and then by a third total reflection prism 230c into the lens 240. The lens 240 then concentrates and focuses its received light onto the image screen 250 to display a composite color image.
FIGS. 5A and 5B illustrate the operation of a light valve. As shown, first and second substrates 310 and 320 having first and second patterned electrodes 330a and 330b are spaced apart from each other, and a liquid crystal 300 is interposed therebetween. When no electric field is induced by the first and the second patterned electrodes 330a and 330b, as shown in FIG. 5A, the liquid crystal 300 maintains its first ordered molecular alignment wherein the liquid crystal molecules are parallel with the substrates.
However, as shown in FIG. 5B, when an electric field is induced between the first and the second patterned electrodes 330a and 330b by a voltage source 350, first portions 300a of the liquid crystal 300 between the first and second patterned electrodes 330a and 330b realign such that the liquid crystal molecules of the first portions are perpendicular to the substrates. In second portions 300b adjacent the first portions 300a the liquid crystal molecule maintain the first ordered molecular alignment. Therefore, under the influence of the electric field, the first and the second portions 300a and 300b of the liquid crystal 300 attain different alignments. Such alignments have different refractive indexes. Thus, the transmission of incident light through the light valve can be controlled according to differences in the refractive indexes of the first and second portions of the liquid crystal.
The liquid crystal beneficially has a fast response time to enable the processing of a large quantity of image data, especially that of moving images. However, in the nematic or the cholesteric liquid crystal, the time required for the molecules to realigned under the influence of the electric field are too long, and consequently the response time of the liquid crystal is not fast enough for many applications.
Because of such limitation, a ferroelectric liquid crystal (FLC) in the smetic phase has become of interest. The FLC has a hundred times faster response time than the TN LC or the STN LC. This is because the FLC has a spontaneous polarization and a bistability that leads to high-speed responses, and thus an improvement in the imaging of moving images. The high speed response of the FLC also improves the operation of a mouse used as an input device in computers, and the operation of window operating systems.
FIG. 6 shows molecular alignments of the FLC. As shown, the longitudinal axes of the liquid crystal molecules gradually align along a helical structure.
To adapt the ferroelectric LC for liquid crystal display applications, the cell gap between the two transparent substrates of a liquid crystal display device should be uniformly maintained at less than about 2 micrometers. However, as shown in FIG. 7, a ferroelectric LC changes phase according to temperature. When compared with a smetic A (SmA) phase at high temperature, lower temperature phases smetic CA* (SmCA*) and smetic C* (SmC*) have the longitudinal axes of their molecules tilted at a tilt angle “θ” with respect to a line that is perpendicular to the substrates (not shown).
Therefore, the molecular layer thickness “d2” of the smetic CA* (SmCA*) phase or the smetic C* (SmC*) phase is less than the molecular layer thickness “d1” of the SmA phase. These thickness differences between the phases of a ferroelectric LC cause difficulty in maintaining a uniform layer spacing.
Further, since the molecular layers of the SmA phase are more ordered than those of the Sm CA* phase or of the SmC* phase, the molecules of the SmA phase are relatively easily aligned with an aligning treatment. Therefore, to control the early state of the molecular alignment in the Sm CA* or SmC* phase, the molecules are conventionally aligned in the SmA phase.
However, after the molecules are aligned in the SmA phase, as the phase of the FLC changes to the SmCA* phase or to the SmC* phase, the molecular alignments become more disordered due to the molecular layer thickness (d1, d2) difference between the phases. That is to say, as the phase changes from the SmA to the SmCA* or SmC* phase, the molecules tilt to a definite angle, and the molecular layers space with gaps between themselves such that the molecular alignments are disordered. Additionally, the high temperature SmA phase has a lower transmittance than the lower temperature SmCA* and SmC* phase. A lower transmittance results in a low luminance of the liquid crystal display panel.